It frequently becomes necessary to provide for equalization of the pressures on opposite sides of a wall while preventing random flow of air therebetween. This situation generally arises in connection with large commercial refrigerators. Opening or closing of the door of such a refrigerator, where the door opens outwardly and closes inwardly, can cause a substantial and undesired change in the pressure within the enclosure. Change in pressure can also be caused by start-up of a compressor. A simple opening or vent port in the wall of the enclosure will not suffice since there will be random flow of air through the port, serious transfer of heat through the port and frost build-up in the port. As is evident, then, a vent port which will permit equalization of the pressures on opposite sides of the wall and which will yet prevent these phenomena is to be desired. A number of devices for achieving these objectives have been constructed and are commercially available. However, conventional devices are relatively complex and relatively expensive, apt to fail after short periods of use and not readily adaptable to variable conditions of use. The present invention is designed to overcome these difficulties.